Continuous on-line measurement of fluid or slurry rheology

ABSTRACT

A rheometer or viscometer for continuously measuring the rheological properties of non-Newtonian, mixed phase fluids or slurries. The device has a vertically disposed, inverted U-tube to maintain solids in suspension and, therefore, exhibit a homogeneous character. The fluid flow rate is controlled to assure nonturbulent laminar flow through the tube. Pressure is measured at as point in each leg of the U-tube and compared. For a constant flow rate, the pressure drop over the tube is used to measure the viscosity of the fluid in the tube.

TECHNICAL FIELD

This invention relates to the continuous measurement of the rheologicalproperties, such as viscosity and yield stress, of fliuds or slurries,especially fluids or slurries which exist in rugged industrialenvironments such as are found in the mining and mineral processingindustries. More particularly the invention relates to the rheologymeasurements of fluids or slurries having significant quantities ofentrained air or gases present and/or coarse (usually up to about 10 mmdiameter) solid particles, varying ambient temperature and/or varyingfluid or slurry temperature and pressure.

BACKGROUND ART

On-line viscometers are known for determining the yield stress andplastic viscosity of non-Newtonian or Bingham type fluids, calculatedfrom formulae for laminar flow. However, in general, such known on-lineviscometers are not capable of handling the extremes or demands whichexist in the mining and mineral processing environments. For example,rotational types often suffer from reliability problems especially whenapplied to slurries. They also have difficulty in handling coarse solidparticles and high process pressures. The static force measurement typeviscometers are not suited to coarse particle slurries and will giveerroneous readings when entrained air is present. Similar comments applyto vibrating element type viscometers.

The general technique of measuring the pressure differential between twopoints in a laminar fluid flowing through a tube of known diameter at aknown rate is well known. This is shown, for example, in U.S. Pat. Nos.3468158, 4680957 and 4384792. A technique using a vertical tube tomeasure slurry viscosity is shown in Australian Patent Application No.31949/77. U.S. Pat. No. 3520179 discloses a rheometer or viscometer fornon-Newtonian fluids using a capillary U-tube type apparatus, howeverthis apparatus does not use differential pressure measurements, andfurther does not appear to be suitable for on-line measurements. U.S.Pat. No. 3465573 also discloses a U-tube type arrangement, but uses acomparison between two different paths, rather than the mere U-tube, toproduce a measurement.

U.S. Pat. No. 4680957 (Dodd) would appear to be the most relevantreference to the present invention in that it discloses a viscositylevel conduit to measure viscosity. However one of the main problemswith this type of viscometer is the tendency for solids to settle out ofsolution along the conduit thus adversely affecting the accuracy ofreadings from the viscometer. There is no suggestion of the use of aU-tube in this specification.

DISCLOSURE OF INVENTION

It is an object of the present invention to overcome the above and otherdisadvantages by the provision of a method and apparatus for the measureof a fluid or slurry rheology or viscosity.

This object may be achieved, in accordance with the present invention,by the provision of a rheometer comprising an inlet for a sample of anon-Newtonian fluid to be measured, a vertically disposed invertedU-tube conduit connected to said inlet at one end and to an outlet atits other end, pressure differential measurement means operativelymounted across the said U-tube and flow meter and control valve meansdisposed between said outlet and said other end of the U-tube.

The present invention further provides a method for measuring andmonitoring the rheological properties of a fluid or slurry flowingthrough a vertically disposed inverted U-tube conduit, comprising thesteps of

(a) providing a stream of fluid or slurry through said U-tube conduit innon-turbulent laminar flow;

(b) determining the pressure differential across the U-tube; and

(c) further determining the rheology or viscosity therefrom.

The invention provides a basis for on-line adjustment of slurryconsistency and rheology by more efficient usage of chemical reagentsand modifiers, resulting in substantial modifiers, resulting insubstantial cost reductions.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the reader may gain a better understanding of the presentinvention, hereinafter will be described a preferred embodiment thereof,by way of example only and with reference to the accompanying drawing

(FIG. 1) which is a schematic representation of an on-line rheometer orviscometer in accordance with the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring to FIG. 1 the rheometer or viscometer 1 comprises an inletconduit 2 being a sub-stream from, and in parallel with, the main slurrytransport conduit (not shown) leading into a vertically disposedinverted U-tube section of the conduit 3 through which the slurry orfluid is caused to flow either by gravity or by pumping. Thedifferential pressure across the U-tube is measured by means of atransducer or transmitter 4 mounted directly onto the U-tube. The flowrate is controlled by means of a magnetic flow meter 5 and a controlvalve 6 under the operative control of a microprocessor 7. The slurry orfluid then passes back via outlet conduit 8 to the main slurry transportconduit (not shown).

The chosen flow rate and the internal diameter of the U-tube 3 are suchthat laminar rather than turbulent flow prevails in the U-tube. For aconstant flow rate the differential pressure across the U-tube 3 is thena measure of the fluid or slurry rheology or viscosity. The U-tube isinverted and disposed in a vertical plane to ensure that the fluid orslurry passing therethrough is thoroughly mixed or homogenous. Inhorizontal flow a laminar stream has little capacity to maintain coarseparticles in suspension. As a result the particles tend to settle in ahorizontally disposed conduit thus increasing the pressure gradient. Avertically flowing stream, on the other hand, has a far greater capacityto maintain coarse solids in suspension, and it is for this reason thatthe U-tube 3 is disposed vertically.

It is common practice to isolate pressure transmitters from the processfluid or slurry by means of an oil-filled diaphragm and capillaryarrangement. However, these exhibit some sensitivity to ambienttemperature which is especially significant at the low differentialpressures measured in the apparatus according to the present invention.To overcome this the pressure transmitter 4 is mounted directly betweenthe two legs of the U-tube 3 with the process fluid or slurry being indirect contact with the diaphragms of the pressure transmitter. Errorsdue to ambient temperature changes are thereby eliminated.

The flow rate is measured by the magnetic flow meter 5 the diameter ofwhich is less than the U-tube 3 so as to generally result in turbulentflow within the flow meter. This ensures the fluid or slurry is ashomogeneous as possible to improve the accuracy of flow measurement.

The flow rate signal is directed to the computer 7 which controls thecontrol valve 6. This valve is of the hydraulically actuatedconstricting muscle type with a concentric round hole construction. Suchvalves are ideal for slurries and will pass coarse particles and give along life. With the 25 mm size valve normally used in the present caseparticles up to 10 mm are easily handled.

In industrial plants significant quantities of entrained air or gasesare often present in the process fluid or slurry. As noted previouslythis can result in erroneous readings in some presently availableinstruments. In the present device air or gas can accumulate in thedownstream leg of the U-tube 3 thereby introducing an additional staticpressure component resulting in errors in measurement. To overcome thisthe computer 7 is programmed to periodically open the control valve 6fully to provide a high flow rate flushing action. The flow rate duringthis flushing period is sufficiently high so as to ensure turbulent flowwithin the U-tube. This turbulence, together with the high velocity offlow, ensures the removal of all air or gas together with any coarsesolids which may have accumulated in parts of the conduit. During theflush cycle the viscometer output signal is "frozen" at the value justprior to the start of the flush cycle.

The frequency and period of the flush can be programmed to suit theparticular plant conditions. The computer processes and averages thesignal from the differential pressure transmitter and displays theresult in the selected rheology units. In the case of mineral slurriestheir rheological behaviour is often described by the Bingham Plasticmodel. For such slurries the differential pressure measured across theU-tube is nearly proportional to the Bingham Yield Stress so the outputcan be displayed in Yield Stress units if desired.

All components used in the present invention, such as the differentialpressure transmitter, the magnetic flow meter and the control valve arewell proven in the harsh environment of mineral processing plants.Maximum reliability is therefore ensured. All of these components arecapable of operation at high process pressures and temperatures. Thereare no moving parts in contact with the process fluid or slurry otherthan the slight flexing of the control valve sleeve. This ensuresreliability and maximum wear resistance.

In summary it is the unique combination of vertical U-tube, directmounted pressure transmitter, periodic flushing action and the use ofproven components which results in an on-line viscometer capable ofmeeting all the extreme demands required of an on-line viscometer.

INDUSTRIAL APPLICABILITY

The on-line viscometer according to the present invention is applicableto the on-line measurement and modification or adjustment of viscosityof all fluids or slurries including those having entrained air or gasespresent and those having coarse (typically up to 10 mm) solid particlespresent. The invention provides a basis for on-line adjustment ormodification of slurry consistency and rheology by more efficient usageof chemical reagents and modifiers normally used in such situations,resulting in substantial cost reductions for the process as a whole. Theviscometer is not affected by ambient or process temperature changes andis capable of measuring viscosity at high process pressures andtemperatures. In this regard and in its tolerence to corrosive fluids itis limited only by the capability of the component parts which areavailable in types to suit most applications.

I claim:
 1. The combination of an apparatus for on-line measurement offluid rheology and a fluid flowing therein, said combinationcomprising:a non-Newtonian fluid containing entrained air and/or coarsesolids, and an apparatus comprising:(a) a substantially verticallydisposed, inverted U-shaped tube forming an upflowing inlet arm and adownflowing outlet arm, (b) means for maintaining a constant rate ofnonturbulent laminar flow through said tube, and (c) means for measuringthe pressure differential between a point in said inlet arm and a pointin said outlet arm of said tube.
 2. The combination according to claim1, wherein said means for maintaining comprises a flowmeter, a controlvalve, and means for opening or closing said control valve in responseto the measurement signal from said flowmeter.
 3. The combinationaccording to claim 2, wherein said flowmeter is a magnetic flowmeter. 4.The combination of claim 3 wherein said magnetic flow meter exhibits adiameter that is sufficiently less than the diameter of said U-shapedtube to result in turbulent flow through said flow meter.
 5. Thecombination according to claim 1, wherein said fluid is a slurry or aliquid having gas entrained therein.
 6. The combination according toclaim 5, further comprising means for purging the apparatus to removeaccumulated gases and solid particles.
 7. The combination of claim 1wherein said means for measuring comprises a pressure transmittermounted between said inlet arm and said outlet arm and providing directcontact between fluid in each arm and diaphragms in said pressuretransmitter.
 8. Apparatus for on-line measurement of fluid rheology,said apparatus comprising in combination:a substantially verticallydisposed, inverted U-shaped tube; means for maintaining a constant rateof flow through said tube wherein said means comprises a flowmeter, acontrol valve, and means for opening or closing said control valve inresponse to a measurement signal from said flowmeter; and means formeasuring the pressure differential between a point in each of the twoarms of said tube.
 9. Apparatus according to claim 8 furthercomprising:means for purging said apparatus to remove accumulated gasesand solid particles.
 10. Apparatus for on-line measurement of fluidrheology, said apparatus comprising in combination:a substantiallyvertically disposed, inverted U-shaped tube; means for maintaining aconstant rate of flow through said tube; means for measuring thepressure differential between a point in each of the two arms of saidtube; and means for purging said apparatus to remove accumulated gasesand solid particles.